Fig. 2. Cephalopod mollusc cardiac NK2 proteins carry the YRD and show a domain structure resembling their mammalian relatives. (A) Homeodomain and YRD sequences from cephalopod and other cardiac NK2 proteins. Tyrosine 54 within the homeodomain, a defining hallmark of NK2-class homeodomains, is shown in red. Tyrosines and phenylalanines within the YRD are shown in red, with other conserved amino acids in green. Species abbreviations: Ag, Anopheles gambiae (mosquito); Am, Apis mellifera (honeybee); amphi, amphioxus; c, chick; Ce, Caenorhabditis elegans (nematode); Cn, cnidarian Hydra vulgaris (hydra); Dm, Drosophila melanagaster; h, human; m, mouse; Nv, Nematostella vectensis (sea anemone); Ok, Octopus kaurna (octopus); So, Sepia officinalis (cuttlefish); x, Xenopus laevis (frog); z, zebrafish. TN, Tin-domain. (B-D) LpNkx2-5 expression in Loligo paelii (squid) embryos. (B) Schematic diagram of a squid embryo showing location of systemic heart (sh; red) and branchial hearts (bh; yellow). (C) Histological section showing systemic heart and ink sac (is). (D) In situ hybridisation to an adjacent section showing LpNkx2-5 expression in the systemic heart and ink sac (arrow), as well as muscle beneath the fins (f; arrowheads). (E) Phylogenetic relationships and domain structure of NK2 protein from divergent species. G, GIRAW motif;N, Nkx2-5 box; NK2, NK2SD.